Based on epidemiological, clinical and pathological data, the existence of two etiologic entities, for what is generically considered as gastric carcinoma, is postulated. Environmental factors would be more important in the etiology of the differentiated type of gastric cancer and genetic factors would have a relevant role in the etiology of the undifferentiated type. Geographic, ethnic, environmental and genetic factors participate in the epidemiology of gastric cancer. This review emphasizes the evidences supporting the role of genetic factors in the etiology of undifferentiated gastric cancer. The importance of looking for a share phenotype among patients with this type of cancer is underscored.